Surface sediments collected from deep slopes and basins (1018–4087 m depth)
of the oligotrophic eastern Mediterranean Sea have been analysed for bulk
elemental and isotopic composition of organic carbon, total nitrogen and
selected lipid biomarkers, jointly with grain size distribution and other
geochemical proxies. The distribution and sources of sedimentary organic
matter (OM) have been subsequently assessed and general environmental
variables, such as water column depth and physical circulation patterns, have
been examined as causative factors of deep-sea sediment characteristics.
Lithogenic and biogenic carbonates are the dominant sedimentary fractions,
accounting for up to 85.4 and 66.5 % of the total weight respectively.
The low OC and TN contents in the surface sediments of the study area, which
ranged from 0.15 to 1.15 % and 0.06 to 0.11 % respectively, reflect
the oligotrophic character of the eastern Mediterranean Sea. Both bulk and
molecular organic tracers reflect a mixed contribution from autochthonous and
allochthonous sources for the sedimentary OM, as indicated by relatively
degraded marine OM, terrestrial plant waxes and anthropogenic OM (e.g.
degraded petroleum by-products) respectively. Wide regional variations have
been observed amongst the studied proxies, which reflect the multiple factors
controlling sedimentation in the deep eastern Mediterranean Sea. Our findings
highlight the role of deep eastern Mediterranean basins as depocentres of
organic-rich fine-grained sediments (mean
The burial of organic matter (OM) in marine sediments constitutes the main link between “active” pools of carbon in the oceans, atmosphere and landmasses and carbon pools that cycle on much longer, geological, timescales (Burdige, 2007). Therefore, investigating the processes that control the composition of sedimentary OM that is buried in deep-sea sediments is crucial for understanding carbon cycling on a global scale.
The deep sea receives inputs of organic particles from multiple sources,
both autochthonous (e.g. biogenic particulate matter from primary
production in ocean surface waters) and allochthonous (i.e. land-sourced OM
from soils, plant debris, riverine phytoplankton and man-made compounds
transported by runoff and atmospheric deposition into the marine domain) (Bouloubassi
et al., 1997; Durrieu de Madron et al., 2000; Kaiser et al., 2014).
Consequently, sedimentary OM constitutes an heterogeneous and complex
mixture of organic compounds with a wide range of chemical and physical
properties (Mayer, 1994; Hedges and Oades, 1997; Hedges et al., 1997; Goñi et al., 1998).
Therefore, the combined use of bulk geochemical indicators such as total
nitrogen (TN) to organic carbon (OC) ratios, stable isotope of OC (
The biogeochemical composition of sediments in deep basins of the oligotrophic eastern Mediterranean Sea (EMS), as well as the sources, transport and preservation of sedimentary OM, have been scarcely investigated so far. Previous studies have shown that the composition of surficial sediments is principally controlled by the geochemical characteristics of the source areas, the prevailing metoceanic conditions on the adjacent shelves, the contribution of atmospheric aerosols and the dominant regional circulation (e.g. Weldeab et al., 2002; Ehrmann et al., 2007; Hamann et al., 2008). Nevertheless, the factors involved in the supply, distribution and fate of sedimentary OM are still poorly known.
In the present study, surface sediments collected from deep slopes and
basins of the EMS have been analysed for physical and geochemical parameters
such as grain size distribution, lithogenic, calcium carbonate (CaCO
The EMS is a land-locked sea with a complex topography including shelves, slopes, ridges, seamounts, trenches and four main basins: the Adriatic Sea, the Ionian Sea, the Aegean Sea and the Levantine Sea (Fig. 1) (Amblàs et al., 2004; Medimap Group, 2007). The Ionian Sea to the west and the Levantine Sea to the east are longitudinally connected and cover most of the EMS area. They are also the deepest basins of the EMS, with the maximum depth (5267 m) located at the Hellenic Trench, south of the Cretan Arc. The Aegean Sea and the Adriatic Sea represent the northern extensions of the EMS. Both are relatively shallow, in particular the Adriatic Sea, which is dominated by a broad shelf and a slope sub-basin shallower than 1200 m. In the Aegean Sea, which has a particularly complex topography with tens of depressions, highs and islands, water depths up to 2500 m are found north of the island of Crete (Amblàs et al., 2004; Medimap Group, 2007). The southern Aegean Sea (Cretan Sea) is the sea area comprised between the Cyclades Archipelago to the north and the island of Crete to the south, which also includes the western Cretan Straits.
The general circulation pattern of the EMS is anti-estuarine, which results from interactions between basin, sub-basin and mesoscale flows (Bethoux, 1979). The EMS communicates with the western Mediterranean Sea through the Sicily Strait, with an inflow of low-salinity modified Atlantic water (MAW) at the upper 100–150 m of the water column (Rabitti et al., 1994; Malanotte-Rizzoli et al., 1997). MAW flows in an easterly direction getting progressively saltier and warmer until it transforms into Levantine intermediate water (LIW) into the Levantine Sea where it sinks to mid depths (Milliff and Robinson, 1992; Lascaratos et al., 1993).
The eastern Mediterranean deep water (EMDW) is a relatively well-oxygenated water mass, likely as a result of the formation and sinking of warm deep-water that ventilates the deepest levels in the EMS (Schlitzer et al., 1991; Roether and Well, 2001; Meador et al., 2010). Waters from the Adriatic Sea (Adriatic deep waters) have been considered as the main contributor of deep and bottom waters to the EMS (Malanotte-Rizzoli and Hecht, 1988). Nevertheless, the Aegean Sea constitutes a sporadically significant contributor to EMDW through the Cretan deep waters, as in the case of the eastern Mediterranean Transient anomaly in the 1990s (Lascaratos et al., 1999; Theocharis et al., 1999). Additionally, the Aegean Sea constitutes a possible secondary source of intermediate waters to the adjacent Ionian and Levantine seas, through outflows across the Cretan Arc straits (Robinson et al., 2001).
Key factors that control the exchanges through the Cretan Arc straits are the thermohaline properties of water masses and mesoscale variability. For example, the Ierapetra anticyclonic gyre, which is located off the southeast corner of Crete (Ierapetra Basin), exhibits a strong seasonal signal that is linked to variations of the outflow across the eastern Cretan Arc straits (Theocharis et al., 1993; Larnicol et al., 2002). Actually, the several permanent and/or recurrent eddies in each of the EMS sub-basins enhance exchanges between continental shelf and slope waters (Robinson et al., 1992; Malanotte-Rizzoli et al., 1997; Millot and Taupier-Letage, 2005), which in turn influence primary productivity and the settling of OM to the deep-sea floor (Danovaro et al., 2010).
Thermohaline circulation and overall environmental conditions make the EMS
one of the most ultra-oligotrophic environments of the world ocean (Psarra et
al., 2000; Krom et al., 2005; Thingstad et al., 2005; Gogou et al., 2014).
Annual primary production in the EMS averages between 121 and
145 g C m
Short sediment cores were collected with a multicorer at 29 stations,
ranging from 1018 to 4087 m water depth, during six oceanographic cruises in
the Ionian Sea, the southern Aegean Sea (Cretan Sea) and the northwestern
Levantine Sea from January 2007 to June 2012 (Fig. 1 and Table 1). Once
onboard, multicores were visually described and sliced at 1 cm intervals.
Sub-samples collected for grain size and elemental and stable isotopic
composition were stored in sealed plastic bags at 4
The grain size distribution of sediment samples was determined using a
Coulter LS230 Laser Diffraction Particle Size Analyzer, which measures sizes
between 0.04 and 2000
The measured particle size spectrum is presented as % volume in a
logarithmic scale, where volume is calculated from particle diameter,
assuming spherical shapes. Results were recalculated to percentages of clay
(< 4
Location, depth and collection date of sediment samples.
January 2007 samples were collected during the M71 (Leg 3) cruise
onboard the R/V
For the determination of total carbon (TC), TN, OC contents and stable
isotopic composition of OC (
In consistency with published data in the Mediterranean Sea we assumed OM as
twice the OC content (e.g. Heussner et al., 1996; Masqué et al., 2003). The inorganic carbon content was
calculated from the difference between TC and OC measurements. Assuming all
inorganic carbon is contained within calcium carbonate, CaCO
Molar TN
The biogenic silica content was analysed using a two-step 2.5 h extraction
with a 0.5 M Na
The lithogenic fraction was estimated by subtracting the concentration of the
major constituents from total dry weight (% lithogenic
The analytical procedure followed for the determination of lipid biomarkers
has been previously presented in detail (Gogou et al., 1998, 2000, 2007). Briefly,
freeze-dried sediment samples were initially solvent-extracted three times
by sonication with a dichloromethane : methanol mixture (
F
The individual lipids were identified by a combination of comparison of
GC-retention times to authentic standards and comparison of their mass
spectral data to those in the literature. Quantification was based on the
GC-MS or GC-FID response and comparison of peak areas with those of known
quantities of standards added prior to the extraction of the sediment
samples ([
Procedural blanks processed in parallel to the samples were found to be free of contamination. Reproducibility of the analytical method based on multiple extractions of sediments was better than 6 % in all cases.
A range of selected lipid biomarkers are considered in this study, namely
long-chain
The sum of the concentrations of the most abundant high molecular weight
The abundance of the UCM of aliphatic hydrocarbons, a commonly observed persistent contaminant mixture in marine sediments consisting of branched alicyclic hydrocarbons (Gough and Rowland, 1990), is used as an indicator of the contribution from degraded petroleum products, i.e. chronic oil pollution in the study area (Wang et al., 1999).
The carbon preference indices of long-chain
Statistical dendrogram of type-averaged grain size profiles and
geographical distribution of grain size compositional types for
Finally, the abundance ratio of
Statistical treatment of grain size data was carried out using the GRADISTAT
v. 8.0 software (Blott and Pye, 2001). Median diameter
(
Principal component analysis (PCA) was performed on standardized grain size
and elemental composition data (% clay and sorting of lithogenic and bulk
fractions, lithogenic, CaCO
The spatial distribution of the various geochemical parameters' contents, bulk OM signatures and selected lipid biomarkers' concentrations/indices considered in this study were visualized using Ocean Data View (Schlitzer, 2011).
Bulk composition and sedimentological parameters of the investigated surface sediments.
Empty cells are not determined.
The grain size composition (% clay, silt and sand) and the sedimentary
parameters (
Silt- and clay-sized particles dominate the bulk sediment, accounting for up
to 76.7 and 57.1 % of the total weight respectively (Table 2). The lowest
values (< 40 %) for the silt fraction are found in the upper
slope of the western Cretan straits (station Red3) and the northwestern
Levantine Sea (station BF19), while the highest values
(> 65 %) correspond to the Ionian Sea (stations H12 and H03).
The lowest clay contents (< 20 %) are also found in the upper
slope of the western Cretan Straits (station Red3) but also in the
northeastern Ionian Sea (station H12), while maximum values
(> 55 %) are recorded at the northwestern Levantine Sea
(station Red1.1 in Ierapetra Basin) and the western Cretan Straits (station
H01). Sand contents show large variations, i.e. from 0 to 47.7 % (station
Red3 in the upper slope of western Cretan Straits), with values less than
2 % in most of the stations (Table 2). Relatively high values
(> 10 %) are also obtained in the northwestern Levantine Sea
(stations Red2, BF19 and BF24).
Sorting of bulk sediment ranges from 3.0 to 5.2 (Table 2). Most of the
northwestern Levantine Sea and western Cretan Straits' stations are very
poorly sorted and all stations within the southern Aegean Sea and most of the
Ionian Sea are poorly sorted (Table 2 and Fig. 2a). Skewness values for the
investigated samples range from
The hierarchical cluster analysis of all bulk sediment samples resulted into
seven grain size types (Fig. 2a). Most of the samples group into cluster
types I (
As in the bulk sediment, silt- and clay-sized particles dominate the
lithogenic fraction, accounting for up to 73.5 and 50.8 % of the total
weight respectively. The hierarchical cluster analysis of the lithogenic
fraction identified six grain size types (clusters) (Fig. 2b). A majority of
samples are highly similar (types I–V), with an average composition of
The spatial variability of lithogenics, CaCO
The lithogenics content in the analysed surface sediments range between 32.5
and 85.4 % (Fig. 3a). Higher percentages (> 70 %) are found
in stations of the Ionian Sea (with the exception of station H02), while the
lowest percentages (< 40 %) are found in the southern Aegean
(stations Red4 and Red5) and northwestern Levantine seas (stations Her01 and
BF19) (Table 2). The CaCO
Opal contents are very low, ranging from below detection limits to a maximum of 0.24 % in the southern Aegean and northwestern Levantine seas (stations Red5 and Red13) (Table 2). Since opal contents are very close to the detection limits, those values can be considered as negligible. Therefore inorganic geochemical fraction of the investigated deep EMS sediments consists only of lithogenic (terrigenous) and carbonate components.
OC contents in the studied samples range from 0.15 to 1.15 %, with an average value of 0.47 % (Table 2). The lowest values are recorded in the northeastern Ionian Sea, south of Otranto Strait (station H12), while the highest values are found off the Gulf of Taranto (station H07), followed by stations in the Ionian Sea (stations BF27, H04 and H03). TN contents range from 0.01 to 0.11 % with an average value of 0.06 %. TN display a pattern similar to OC, also with the highest values recorded off the Gulf of Taranto (station H07) and the lowest south of Otranto Strait (station H12), both in the northern Ionian Sea.
The spatial distribution of molar TN
Spatial distribution of
Spatial distributions of the OC-normalized concentrations of
Molar TN
The spatial distribution of
The analysed sedimentary aliphatic hydrocarbons comprise of a series of
resolved compounds, mainly
The molecular profile of the
The aliphatic alcohol fraction is dominated by a series of
Concentrations (OC-normalized) and indices of the considered lipid biomarkers.
Empty cells are not determined;
Long-chain di- and tri-unsaturated C
Three main principal components (PCs) are identified from PCA, accounting for
64.3 % of the variation within the data set (23.8, 22.8 and 17.7 %
for PC1, PC2 and PC3 respectively). PC1 is characterized by positive
loadings for water depth,
Factor scores on each PC display significant variability amongst the studied stations, both within the same area and from one area to another (Fig. 5b). High positive factor scores on PC1 are observed both in stations to the west (Ionian Sea) and east (western Cretan Straits and northwestern Levantine Sea). For PC2, an eastward increasing contribution of positive factor score values seems to exist, with the highest ones located in the southern Aegean Sea and the northwestern Levantine Sea. In contrast, the prevalence on PC3 is recorded in stations of the Ionian Sea and in parts of the northwestern Levantine Sea (Ierapetra Basin).
The contents of CaCO
Surface sediments of the Ionian Sea show a significant (
Terrestrial lipid biomarkers concentrations (
Clearly, the surface sediments of the deep EMS mostly consist of lithogenics and carbonates, with low OC contents while opal is nearly absent (Table 2). The range of lithogenics, carbonates and opal contents recorded in the investigated samples are similar to those previously reported for the eastern Mediterranean Sea (Emelyanov and Shimkus, 1986; Bethoux, 1989; Cros, 1995; Kemp et al., 1999; Rutten et al., 2000; Struck et al., 2001). OC contents reach values slightly above 1 % and are also comparable to those found in the eastern Mediterranean Sea (0.56–1.51 %, Danovaro et al., 1993; 0.23–0.99 %, Bianchi et al., 2003; 0.30–0.82 %, Gogou et al., 2000; 0.25–1.73 %, Polymenakou et al., 2006) and relatively lower than those found in the western Mediterranean Sea (0.80–1.60 %, Kaiser et al., 2014; 0.47–1.53 %, Masqué et al., 2003; 0.23–1.85 %, Roussiez et al., 2006). Values found are comparable to the typical hemipelagic sediments found in continental slopes (Rullkötter, 2006) and slightly higher than those in deep basin areas (Seiter et al., 2004).
The grain size of the lithogenic fraction found in the studied sediments is
very similar to that of Saharan dust particles, which mainly consist of
clayey silts and silty clays with diameters ranging from 0.5 to 60
Riverine inputs have a rather minor influence onto the open EMS sedimentation as they are small and localized (Weldeab et al., 2002; Statham and Hart, 2005). The relatively higher lithogenic contents found in most of the Ionian Sea stations (Fig. 3a and Table 2) points to fluvial inputs reaching the area from the Adriatic Sea. The main source of riverine inputs is the Po River, opening into the northernmost end of the Adriatic Sea, although inputs from smaller rivers draining the Apennines could be also relevant (Weldeab et al., 2002). In the Ionian Sea, river-sourced particles are carried by both surface and deep currents flowing southwards along the Italian Peninsula as part of the overall anticlockwise circulation in the Adriatic Sea (Orlic et al., 1992). It should be noted that dense water formation takes place seasonally in the Adriatic Sea, which triggers episodes of fast-flowing, sediment-loaded dense near-bottom currents that cascade into the deeper Meso Adriatic depression before passing through the Otranto Strait, subsequently spreading into the Ionian Sea where their particle load settles to the bottom (e.g. Zoccolotti and Salusti, 1987; Manca et al., 2002; Canals et al., 2009).
The grain size variability of the carbonate particles recorded in the
studied sediment samples is indicative of calcareous skeletons of primary
producers. While the abundance of particles < 8
Bulk geochemical proxies such as elemental (TN
Marine-derived OM is characterized by high TN contents yielding TN
In order to constrain the origin of sedimentary OM and assess the spatial
variability in its marine-to-terrestrial blend molar TN
Additionally, the relative contribution of the marine vs. terrestrial
sources of OC over the study area has been evaluated by means of a simple
As evident in Table 2, sediments from the Ionian Sea are characterized by
elevated contributions of terrestrial OC (OC
Lipid biomarkers have often been used as molecular proxies to identify specific biological precursors of sedimentary OM (Meyers, 1997; Volkman, 2006). The concentrations of the sedimentary lipid biomarkers determined in this study are fairly comparable to those previously reported in areas devoid of significant fluvial influence both in the eastern and western Mediterranean basins (Grimalt and Albaigés, 1990; Gogou et al., 2000; Gogou and Stephanou, 2004; Kaiser et al., 2014).
The patterns of long-chain
Lipid biomarkers preserved in the surface sediments of the study area also
highlight the contribution from autochthonous marine OM derived from in
situ phytoplankton production. More specifically, the abundance of
brassicasterol (
In addition, while the abundance of cholesterol (
Aside from natural sources, the abundance of UCM indicates a contribution of anthropogenic OM resulting in chronic oil pollution of the investigated sediments (Parinos et al., 2013). UCM levels recorded in the deep EMS are comparable to those reported for surface sediments in unpolluted coastal and/or open-sea areas and are at least one order of magnitude lower than those reported for coastal areas subjected to enhanced anthropogenic inputs (Gogou et al., 2000; Parinos et al., 2013; Kaiser et al., 2014; Romero et al., 2015; and references therein). Two main pathways have been identified for the introduction of petroleum hydrocarbons into the deep EMS, which are direct discharges from merchant shipping and oil transportation (UNEP, 2010) and atmospheric transport and deposition (Gogou et al., 1996; Castro-Jiménez et al., 2012; Parinos et al., 2013).
The PCA provides a robust overview of the variables and processes controlling the geochemical composition of the investigated deep-sea surface sediments (Fig. 5).
The significant positive loadings of
The second PC separates samples with high carbonate contents, molar TN
Finally, PC3 separates samples with high contents of OC, TN and clays from
those with high values of
The low OC and TN contents in the surface sediments of the study area
reflect the oligotrophic character of the EMS (e.g. Krom et al., 2003). In the studied
sediments some processes may have further pushed TN to OC ratios towards low
values (Fig. 6a). These include the preferential degradation of N-rich
proteinaceous components of algal OM during early diagenesis (Meyers et al., 1996; Meyers,
1997; Hopmans et al., 2004) and the enrichment of OC relative to TN due to
the input of petroleum residues (Friligos et al., 1998).
Furthermore, a significant contribution of inorganic N, presumably as
NH
Although isotopic fractionation specifically associated with early
diagenesis is negligible and the isotopic composition of sedimentary OM is
fairly conservative (e.g. Di Leonardo et al., 2009 and references therein),
Distribution of OC content and bulk sediment tracers (TN
Lipid biomarkers provide further information on the natural sources of
sedimentary OM. The significant positive correlation of
While a significant positive correlation is observed for
The second and third PCs of the PCA highlight the main processes that affect sediment dispersal and deposition in the study area. These relate to pelagic settling of marine skeletons from surface waters (corresponding to PC2) and hydrodynamic sorting of organic-rich fine sediment by bottom currents (corresponding to PC3).
Particulate matter exported from the upper layers of the water column in the EMS is primarily composed of biogenic particles and atmospheric dust, which while settling to the seabed are able to transfer OC, other nutrient elements and OC-associated organic pollutants (e.g. Stavrakakis et al., 2000, 2013; Theodosi et al., 2013). In the deep EMS, the distribution of pelagic carbonates (second PC) seems to be mainly influenced by planktonic contributions. In the study area, the phytoplankton biomass and primary production are relatively higher in regions of cyclonic water mass circulation. The Rhodes cold-core gyre, situated in the southeast of the Rhodes Island (NW Levantine Sea), is the most prominent dynamic feature in the EMS and is the main source area of the LIW. In this cyclonic gyre, which is enhanced during winter period, dense water masses from deeper layers tend to upwell at its centre, feeding the upper layers with nutrient-rich waters (Salihoğlu et al., 1990). Therefore, this gyre plays an important role in the productivity of the Levantine Sea.
Spatial distribution of lipid biomarker indices
The third PC separated samples with high OC, TN and clay contents, which is
indicative of a close OM–mineral association. This is in agreement with the
high OC contents found in the fine-grained sediment samples from the deeper
stations representing an essentially quiet environment (Figs. 2, 3 and 7).
This is in contrast to the lower OC contents observed in coarser samples
(
Moreover, in the EMS these bulk sediments show a poorer sorting than the lithogenic fraction (Fig. 2). This could be related to the presence of coarse biogenic carbonate particles in bulk samples or the effective hydrodynamic sorting linked to the prevailing depositional conditions in such deep low-energy environments (Friedman, 1969).
The poor sorting and positive skewness found in grain size types I, II, IV
and V of the bulk sediment samples (Fig. 2a) in the southern Aegean Sea and
the northwestern Levantine Sea are explained by the prevalence of pelagic
biogenic sedimentation, as shown by the high positive score values observed
on PC2 (Fig. 5b). Accordingly, high percentages of CaCO
Surface sediments in the Ierapetra Basin (stations Red15, Red1.1 and Ier01) also show positive scores on PC3 (Fig. 5b), which point to an influence of hydrodynamic sorting processes. The relatively high OC content in these stations (Fig. 7), along with the elevated values of the associated natural and anthropogenic lipid concentrations (Table 3), suggests that the Hellenic Trench is a sink of OC associated to fine particles transferred by the active outflows of the Cretan Straits, besides the pelagic sedimentation related to the well-known semi-permanent Ierapetra anticyclone (Larnicol et al., 2002; Taupier-Letage, 2008).
Sediments with grain size types I–III of the bulk sediment samples (Fig. 2a)
in the Ionian Sea show lower CaCO
Finally, station Red3 from the upper slope of western Cretan Straits
representing grain size type VII of the bulk sediment samples (Fig. 2a)
shows the highest contents of sand (47.7 %), which is poorly sorted. This
is in agreement with the occurrence of the topographically restricted deep
outflow of the western Cretan Straits. These straits are characterized by
maximum outflow speeds during winter and minimum speeds during fall (Kontoyiannis et al., 2005).
The turbulent, fluctuating outflow current should normally trigger sediment
resuspension and induce selective transport, thus leaving coarse OC-poor
particles in the upper slope of the western Cretan Straits (negative factor
scores of PC2 and PC3) and carrying fine OC-rich particles to the lower
slope. A similar pattern has been also observed in other submarine canyon
settings of the Mediterranean Sea, such as the Cap de Creus Canyon
(Sanchez-Vidal et al., 2008) and the Blanes Canyon
(Pedrosa-Pàmies et al., 2013). The top
Surface sediments collected from deep basins of the oligotrophic EMS were investigated using a multi-proxy approach that involved elemental composition, grain size, stable isotopes and selected lipid biomarkers' analyses resulting in a robust database to determine sediment sources, the degradation and preservation state of OM and processes that affect sediment dispersal and deposition. The PCA analysis helped to identify the main controlling factors of the observed geochemical variability in the investigated sediments. Such factors are sediment sources in terms of allochthonous vs. autochthonous, a highly variable physiography, the thermohaline structure and the regional and local circulation, leading to hydrodynamic sorting and regulating particle settling/deposition and OM preservation state.
Surface sediments of the investigated part of the EMS mostly consist of airborne lithogenic particles and biogenic carbonate particles, the latter deriving from primary production into surface waters. Sedimentary OM appears in rather low contents (0.15–1.15 % OC), with bulk and molecular organic tracers reflecting a mixed contribution from both natural (autochthonous and allochthonous) and anthropogenic sources. Samples from locations in the Ionian Sea and the western Cretan Straits that are under the direct influence of the Adriatic dense waters outflow through the Otranto Strait and of currents exiting the southern Aegean Sea respectively are appreciably sorted. Current regime impacted not only grain size but also OC loadings within each subregion of the study area, with winnowing of fine OC-rich particles to the deepest EMS. In contrast, coarse OC-poor particles tend to occur in upper slope settings. While OC associated to fine particles was relatively non-degraded terrestrial OM, marine OM was found to be mostly degraded and reworked during transport processes and before reaching the deep seafloor.
The spatial variability in the yields of sedimentary OC and lipid biomarkers presented in this study highlights the heterogeneous nature of the particle load exported to the deep basins of the eastern Mediterranean Sea. Such variability must be taken into account during the development of quantitative carbon budgets for this area.
This research has been supported by the EU-funded project PERSEUS (GA
287600), the EU–Greek co-funded project KRIPIS (MIS 451724; NSRF) and REDECO
(CTM2008-04973-E/MAR), BIOFUN (CTM2007-28739-E) and MEDECOS
(